I'm a California based audio technician for 26 years.

Beyonce sang a small part of her song then switched to a pre-recorded track start from "Now everybody asks me why I'm smiling out from ear to ear.", which means yes, she lip-synced. At the end She was shaking her head and mic up and down but I didn't hear any wind contortion on the audio. plus her voice was too perfect and echo-free unlike others who didn't lip-sync.

I'm not saying that Beyonce is a bad singer. Moreover, Lip-syncing needs a skill and practice just like the singing itself, the main purpose of lip-syncing is to minimize the risk of unnecessary sound excretion (breath, instrument A-weighting etc.) Beyonce lip-synched from one show to another, and from what i've seen she's the best mimers in the industry. Chris brown in the other hand, mimed to his original recording, which made his lip-synching way more obvious.